The present invention relates to a silicon-nitride-inserted piston which is meant for an internal combustion engine consisting of a composite structure obtained by insert-casting of a silicon nitride and a metallic material. Some of the advantages of the invention include low manufacturing cost of the piston, as well as a simple manufacturing process, and at low temperatures, only a low level of stress is generated in the silicon nitride. Even at high temperatures, the joint between the silicon nitride and the metal body maintains a high level of strength, or in other words, a high level of strength reliability, thereby, being free from breakage during production.
In recent years, active research and development has been conducted regarding mechanical structure components which utilize the excellent heat resistant, wear resistant and heat-insulating properties of silicon nitride. Since silicon nitride is more brittle than metal, it is often difficult to use silicon nitride as the sole material that forms a structural component and it is generally used in the form of a composite body, in combination with a metal.
Methods known for joining a silicon nitride and a metal include: joining them through shrink-fitting, force-fitting or brazing. All of these methods, however, require the sintered silicon nitride body to be ground with a diamond grindstone or the like so that the surface of the body is finished with a high level of precision. The problem arises during this last process which requires a high production cost.
In order to cope with the above problem, it has been proposed to achieve joining by insert-casting, a process which does not require the grinding of the outer periphery of the silicon nitride. However, an insert-casting employing an aluminum alloy, a common piston material, entails a problem in that the aluminum alloy has a greater coefficient of thermal expansion than the silicon nitride, resulting in loosening or gap formation occurring between the silicon nitride and the metallic material, when exposed to high temperatures.
In order to avoid this problem, it has been proposed to conduct an insert-casting by employing a metallic material composed mainly of iron, having a smaller coefficient of thermal expansion than an aluminum alloy. However, an iron-containing material has a melting point higher than that of an aluminum alloy, thereby presenting another problem. The pouring temperature during insert-casting is inevitably raised, causing the generation of excessive stress in the silicon nitride during the insert-casting, and thus increasing the risk of breakdown of the silicon nitride.
The present invention is directed toward overcoming the conventionally entailed problems, solvable by providing a silicon-nitride-inserted piston for an internal combustion engine consisting of a composite structure obtained by insert-casting of a silicon nitride and a metallic material. Machining is unnecessary for the joining of the silicon nitride and the metallic material, hence the composite is producible by a small number of production processes at a low production cost and the silicon nitride is hardly vulnerable to breakage during insert-casting.